1. Field of the Invention
The present invention relates to an optical ring network based on a wavelength division multiplexing (WDM) technology, and more particularly to an optical ring network capable of transmitting optical signals in a two-way direction and performing self-healing, and its related method of providing an efficient recovery switching of transmission signals when a link failure occurs.
2. Description of the Related Art
With the commercialization of wavelength division multiplexing (WDM) technology for transmitting multiple signals with different wavelengths through a single optical fiber, it is possible to send a plurality of high-capacity optical signals at a very high speed. Further development in techniques in routing, switching, and dropping/adding of optical signals now allow designers and engineers to build an optical network based on the WDM technology.
The WDM optical networks may be generalized into a ring network based on an optical add/drop multiplexer and a mesh network based on optical line distributor. Both of these optical networks require an efficient scheme to cope with a link failure. In the mesh network, a protective switching against such a failure is slow but not complicated as each node is connected to a plurality of optical fibers. In the ring network, it is easier since the optical add/drop multiplexer constituting a node is connected with two or four optical fibers. For this reason, the ring network is more preferred and widely used.
In the WDM optical ring network, each node may include at least one optical add/drop multiplexer for dropping or adding optical signals and at least one switching device for performing a protective switching of signals. According to a protective switch technique, the ring networks may be classified into networks based on a path protection switch scheme and a link protection switch scheme. Further, the ring networks may utilize two or four optical fibers depending on the number of input/output optical fibers, and are classified into a unidirectional transmission and a bi-directional transmission depending on the data transmission direction.
FIG. 1 shows a configuration illustrating the protection switching technique of a bi-directional optical network according to the conventional technique. As shown in FIG. 1, each node of the ring network comprises a plurality of optical add/drop multiplexers 10a to 40a; and 10b to 40b, each of which adds or drops individual optical signals, and a plurality of 2×2 optical switching devices 110 to 180, each of which performs the switching of signals through inner and outer rings. In the outer ring network 4, optical signals with wavelengths of λ1, λ2, λ3, . . . , λN are transmitted. In the inner ring network 2, optical signals with wavelengths of λN+1, λN+2, λN+3, . . . , λ2N are transmitted. In this case, the outer ring network 4 is for a clockwise transmission, while the inner ring network 2 is for a counter-clockwise transmission.
When a link failure occurs, the optical network performs the protection switching by transmitting the optical signals in the looped-back or inverse direction using two of the 2×2 optical switching devices, which are arranged on both ends oft he failed link. For example, referring to FIG. 1(b), when the failure occurs at the outer fiber optic link connecting between the first outer optical add/drop multiplexer OADM1a or 10a and the second outer optical add/drop multiplexer OADM2a or 20a, optical signals with wavelengths of λ1, λ2, λ3, . . . , λN, which are sent from the first outer optical add/drop multiplexer OADM1a or 10a to the second outer optical add/drop multiplexer OADM2a or 20a, are looped-back to the first inner optical add/drop multiplexer OADM1b or 10b through the second-right optical switching device sw12 or 120, and then transmitted in the counter-clockwise direction through the inner ring network 2. The optical signals with wavelengths of λ1, λ2, λ3, . . . , λN, which are sent through the inner ring network 2 are transmitted from the second inner optical add/drop multiplexer OADM2b or 20b to the second outer optical add/drop multiplexer OADM2a or 20a through the second-left optical switching device sw21 or 130, and thus completing the protection switch switching.
When the ring network is operated normally, the 2×2 optical switching devices 110 to 180 maintain a bar state or no cross state, so that signals applied to the outer left input port i1 are transferred to the outer right output port o1, and the signals applied to the inner right input port i2 are transferred to the inner left output port o2. However, when a link failure occurs, the optical switching devices 110 to 180 maintain a cross state, so that signals applied to the outer left input port i1 are transferred to the inner left output port o2, while signals applied to the inner right input port i2 are transferred to the outer right output port o1. In FIG. 1(b), the second left optical switching device sw21 or 130 is maintained in the cross state. Thus, the optical signals passing through the failed link are looped-back together with the optical signals with wavelengths of λN+1, λN+2, λN+3, . . . , λ2N, which are sent in a counter-clockwise direction from the second inner optical add/drop multiplexer OADM2b or 20b to the first inner optical add/drop multiplexer OADM1b or 10b. Then, the looped-back optical signals are transmitted in the clockwise direction through the outer ring network 4, and subsequently transferred from the first outer optical add/drop multiplexer OADM1a or 10a to the first inner optical add/drop multiplexer OADM1b or 10b through the first-right optical switching device sw12 or 120. Meanwhile, other optical switching devices of the nodes which are not adjacent to the failed link continue to maintain the bar state without any change.
In the above-described conventional method, there are drawbacks when the bi-directional self-healing optical ring networks are put into practice with two optical fibers. First, when the channel interval of the transmission optical signals becomes narrow, signals are subjected to a deterioration caused by a non-linear property of the optical fiber. Second, because the prior art does perform multiplexing and demultiplexing of the loop-back signals and existing transmission signals simultaneously so as to perform the protection switch when the failure occurs, the multiplexer/demultiplexer must have twice the processing capacity necessary to perform the actual transmission.
FIG. 2 shows a structure of the conventional optical line distributor. It is assumed that an outer optical add/drop multiplexer is operated as one component in each node of the outer ring network 4, and an inner optical add/drop multiplexer is operated as one component in each node of the inner ring network 2. Therefore, each wavelength processed on the outer ring is λ1, λ2, λ3, . . . , λN, while one processed on the inner ring is λN+1, λN+2, λN+3, . . . , λ2N. However, with the outer optical add/drop multiplexer 10a, either a multiplexer 14 or a demultiplexer 12 must have a capacity in consideration of the looped-back transmission signals, so that signals with wavelengths of λN+1, λN+2, λN+3, . . . , λ2N can be multiplexed or demultiplexed while passing through the multiplexer 14 or the demultiplexer 12. Similarly, with the inner optical add/drop multiplexer 10b, signals with wavelengths of λ1, λ2, λ3, . . . , λN must be subjected to multiplexing and demultiplexing. As such, the multiplexer and demultiplexer of the optical add/drop multiplexer for transmitting N optical signals must each have a capacity of 1×2N.
Another drawback is that signals not related to the link failure are also looped-back together. Referring back to FIG. 1, optical signals with wavelengths of λN+1, λN+2, λN+3, . . . , λ2N which travel the normal link of the inner ring network 2 are looped-back when the link failure occurs, and then transmitted along the outer ring network 4 in the clockwise direction. Accordingly, there occurs a problem in that interruption of transmission signals as well as loss of data is incurred.